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    <title>Document</title>
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<script>
    // 数组原型方法的 this 指向调用者 - 传入的数组
    Array.prototype.sum = function() {
        let sum = 0;
        for (let i = 0; i < this.length; i++) {
            sum += this[i];
        }
        return sum;
    }

    // 10
    console.log([1,2,3,4].sum());
</script>
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